The invention in general relates to the purification of wastewater by ultrafiltration and in particular concerns a novel system that permits a large flux of purified water per unit area of membrane, while operating with low pressures and employing filter tubes of relatively large diameter and length. The system also permits continued operation in the event of failure of individual elements of the system and/or while the system is being repaired, and consequently water purification standards may be maintained without interruption over indefinitely long periods of time.
Recently state and federal wastewater discharge regulations and a new awareness of the need for maintaining an unpolluted environment has increased the importance of removing the impurities from wastewater. In addition, the rising price of raw materials has in some cases, made practical the removing of these materials from wastewater and recycling them. These incentives toward improved water purification systems have been particularly cogent in the metal plating and finishing industries because of the high toxicity of heavy metals and heavy metal compounds as well as the relatively high commercial value of these materials. Thus a system that is particularly efficient and effective in removing metal impurities from wastewater has been actively sought by many researchers.
Many methods of water purification have been devised. One method commonly used in purifying wastewater from metal plating and finishing operations is to treat the wastewater with chemicals so that insoluble compounds containing the metal impurities are formed and precipitated. The precipitate is then separated from the water by a variety of means. A major disadvantage of this method is that some heavy metal ions and other contaminants found in wastewater from metal finishing and plating operations are soluble to some extent even in their altered chemical forms, so wastewater treated in this manner often still contains undesirably high levels of impurities. Another more recent method of treating wastewater from metal finishing and plating operations is the ion exchange method. This method has the disadvantage that it is complex and expensive.
Another method suggested for purifying wastewater from a large variety of sources, is to treat the waste with a metal salt such as ferric sulfate, ferrous sulfate, or aluminum sulfate which, if the pH of the water is suitably adjusted, is thought to form a metallic hydroxide which is very effective in binding impurities into particles of a size that can be more easily precipitated, filtered, or otherwise separated from the water. This method has been tried in combination with separating out the resulting precipitate as in conventional water purification systems conbining settling, filtering and chemical treatment, (see U.S. Pat. No. 3,677,405). this method arguably might be suggested for use in the relatively new membrane filtration process with or without settling and chemical treatment (see for example A. S. Michaels, New Separation Technique for The CPI, Chemical Engineering Progress, December 1968 pp. 31-43). However, it is known that metal hydroxides as conventionally used tend to clog ultrafiltration membranes. Thus prior workers have suggested avoidance of iron hydroxide in ultrafiltration systems.
When metal hydroxides were used previously, the amounts used tended to form gelatinous precipitates. It previously was thought that increasing the concentration of metal hydroxide beyond the point at which substantially all the impurities were bound, would not only be a needless waste of the salt which formed the hydroxide, but it would also serve to increase problems associated with filtering increased amounts of the precipitate created by the addition of the salts. The problems created by this pricipitate were thought to impose critical limitations on conventional water purification methods.
It was generally thought that the metal salts combined with water to form a flocculent or gelatinous (as opposed to crystalline) precipitate. This was in fact the very property of the metal salt on which the water purification methods using such a salt were based; that is, the same properties of the metal salt that enabled it to bind a wide variety of impurities, i.e. the strong van der Waals forces, hydrogen bonds, and zeta potentials, also caused it to form a gelatinous precipitate. In one known water purification method the gelatinous precipitate is allowed to settle in a settling zone. A small amount of the precipitate is allowed to enter the subsequent filter beds in these systems because it creates a gelatinous film over the filter which aids in removing suspended matter and bacteria. However, adding metal salts in excess of the amount required to bind substantially all the impurities is contraindicated in the conventional process not only because of the cost of the material, but because additional metal salts only increase the amount of precipitated sludge which must be disposed of, or if the precipitate is allowed to accumulate in the filter bed it will only advance the time at which the filter bed must be removed from service to be rehabilitated.
Membrane filtration is usually considered to be divided into two different processes which create different problems; i.e., the filtration of microsolutes (particles less than ten angstroms), called reverse osmosis, and the filtration of macrosolutes and suspended particles of a size larger than ten angstroms, called ultrafiltration. Both these processes are limited by a phenomenom known as concentration polarization--the buildup of concentration of the solute or suspended matter near the membrane wall as compared to the concentration in the solute stream. Also the precise limitations of both processes depend on whether the fluid being filtered is in turbulent or laminar flow. The present invention relates to the process called ultrafiltration.
It is an important object of this invention to provide a water purification system that utilizes metal hydroxide to bind impurities but does not require settling, prefiltering, or centrifuging prior to the ultrafiltration process.
It is another object of this invention to achieve the preceding object in a water purification system that provides flux rates through the ultrafiltration membrane higher than 30 gfd in a low pressure turbulent flow system.
It is a further object of this invention to achieve one or more of the preceding objects in a water purification system that employs ultrafiltration tube runs no longer than about 12 feet at tube diameters of from 1/2 to 2 inches.
It is another object of this invention to achieve one or more of the preceding objects in a water purification system that can filter commercially large quantities of wastewater but need not be designed to close tolerances.
It is a further object of this invention to achieve one or more of the preceding objects in a water purification system that can filter large quantities of wastewater but is relatively inexpensive to build.
In addition to the requirement of being able to efficiently process large quantities of water, a commercial system must meet a number of other criteria. A shut-down of a water purification system would present the user with the dilemma of either breaking the law by emitting water waste in excess of federal and state standards, or shutting down the manufacturing operations that produce the waste. Therefore, it is essential that any water purification system be capable of operation without interruption for a long period. Since any system will require some repairs and maintenance it is highly desirable that the system be designed so that repairs and maintenance can be performed without shutting down the complete system.
An ultrafiltration purification system presents some additional design problems. If the ultrafiltration membranes are allowed to dry out they irreversibly consolidate and lose their permeability. Thus they must be maintained in a wet state at all times. Also ultrafiltration membranes are extremely thin and large membrane areas must be maintained for any commercial scale operation. Therefore the probability of a leak developing somewhere in a large commercial system must be significant. Thus for the system to be reliable it is essential that any given area of the ultrafiltration membrane can be isolated without shutting down the remainder of the system. In addition, a relatively small leak for a short period can contaminate the filtered water with impurities in excess of legal limits. Thus it is essential that a leak anywhere in the system can be quickly detected and its location pinpointed. Furthermore, it would be preferable that the filtered water from a given small filter area be capable of being isolated from the filter water as a whole; otherwise a small leak would lead to the necessity of re-filtering a very large quantity of water.
Accordingly, it is a further object of this invention to achieve one or more of the objects listed above in a water purification system that is very reliable.
It is another object of this invention to achieve the preceding object in an ultrafiltration water purification system in which the filter membranes can be maintained in a wet condition at all times.
It is another object of this invention to achieve one or more of the preceding objects in a water purification system in which only a part of the filtered water will be contaminated by a leak in any given area of the filter.
It is yet another object of this invention to achieve one or more of the preceding objects in a water purification system in which leaks or other defects can be quickly detected, isolated, and repaired without shutting down the whole system.
It is still another object of this invention to provide a filter tube structure which provides for maximized filtering in a turbulent flow system with maximized exposure of flow to a membrane surface.